The use of superconductive devices, and particularly Josephson tunnelling devices, in sampling or A/D circuits is known in the art, as exemplified by U.S. Pat. Nos. 3,983,419 (Fang) and 3,764,905 (Zappe). Because a Josephson device is capable of extremely fast switching speed between two stable states, and because the device responds to extremely small magnetic fields, use of a Josephson device provides a very sensitive detector offering the possibility of very fast sampling speeds. To date, Josephson sampling circuits are among those providing the highest resolution, because of these advantages.
U.S. Pat. No. 3,983,419 describes a sample and hold technique in which a single Josephson junction device is utilized, and provides an extensive discussion of the background for use of Josephson tunnelling devices in sampling circuits. The Josephson device in that patent is connected in a superconductive loop containing distributed inductance. A control loop is used to switch the Josephson device between its zero voltage state and its finite voltage state.
In U.S. Pat. No. 3,764,905, another sample and hold circuit is described in which two Josephson junctions are used, where each is connected in a superconductive loop. A first loop captures magnetic flux quanta from the unknown signal during a predetermined amount of time. When the Josephson device in the first loop is switched to its zero voltage state, the flux penetrating the loop is trapped and a current will flow in the superconductive loop. This persistent current is used as a control current to switch the state of a test Josephson device in a second loop in order to read the amount of flux quanta (and thereby the amplitude) due to the unknown signal.
These approaches to unknown signal measurement rely upon sampling of the signal and trapping magnetic flux in a superconducting loop containing a Josephson junction device. In such an approach, it is desirable that the acquisition time (i.e., the time required for the superconducting circuit to be updated to the current value of the sampling current) be sufficiently fast that the measured signal can accurately track the applied analog signal. The L/R time constant of the superconducting loop is the primary determinent of the acquisition time. Additionally, the integrity of such a system depends on the design of each loop, since it is often possible to switch the state of one loop while not switching the state of another loop which was intended to be switched. Still further, these approaches require commercial sampling oscilloscopes and high speed cables, which affect the resolution of these circuits.
Another disadvantage of these circuits is that the unknown signal must cross several loops which causes degradation of the signal. Also, trapped flux quanta from previous sampling cycles can present uncertainty. Noise and random events can cause loops to switch in a random fashion rather than in a preselected, well defined sequence.
In order to provide a sample and hold technique in which the critical time is not that for magnetic flux to penetrate a loop, C. A. Hamilton has proposed an approach which does not require a superconducting loop. This approach is described in more detail in copending application Ser. No. 853,354, filed Nov. 21, 1977, and also described in a bulletin NTN 78/0328, "High-Speed Analog Current Sampler", provided by the National Technical Information Service of the U.S. Dept. of Commerce, and in Applied Physics Letters 35, 718 (1979).
In the Hamilton circuit, superconducting loops are not used to trap magnetic flux from an unknown signal. Instead, a first Josephson device is used to produce very short sampling pulses and a second Josephson device acts as a latching type amplitude discriminator. The short output pulse from the first Josephson device is used as a control current to change the state of the second Josephson device, through which the unknown electrical signal passes as a gate current. A bias current is used as a control signal along with a sampling pulse. The bias current is increased or decreased with each short trigger pulse until the sum of the bias current and the trigger pulse is enough to change the state of the second Josephson device. Since the maximum Josephson current through the second Josephson device and the sampling pulse current have constant amplitudes, the magnitude of the bias current is related to the magnitude of the unknown signal.
The circuit of Hamilton is limited by the fact that many samples must be made at a given point in a repetitive waveform in order to resolve the analog signal level at that point. This is stated to be a design choice, because it is considered that a short acquisition time is more important than minimizing the number of required samples.
In that circuit, a sampling oscilloscope is used to establish a time reference since none is provided by the circuitry itself. This means that the resolution and sensitivity of the system are dependent upon that of the commercial sampling oscilloscope and, consequently, there is little improvement over the prior art. Further, since an oscilloscope is required, special, high speed coaxial cables are also required to deliver the output of the second Josephson device to the oscilloscope. Since these cables tend to degrade the output signal due to dispersion of different frequencies of the signal, the resolution and sensitivity of that system are limited. For example, the sensitivity is limited by the sensitivity of the commercial sampling oscilloscope. This means that the second Josephson device is constrained to provide an output in the millivolt range in order to be detectable by the oscilloscope. In turn, this means that the monitor gate cannot be made extremely small to minimize its resistance and capacitance in order to provide the fastest switching of the monitor gate. Still further, that circuit can measure only rise time, rather than both rise and fall times.
The circuit of Hamilton also does not provide for noise cancellation in order to eliminate inaccurate signals due to random events, such as those due to stray magnetic fields. Moreover, it would be difficult to do so, since the step bias approach used therein makes it very difficult to use an averaging scheme to eliminate noise. Noise cancellation is required for ultra high accuracy and it is preferable to provide it in a self-contained circuit in order to more accurately track the unknown signal, and to define the switching threshold unambiguously.
Accordingly, it is an object of the present invention to provide a sampling technique in which high speed cables and conventional sampling oscilloscopes are not required.
Hamilton's circuit requires two pulse generators which have to be designed with tight tolerances, in order to prevent unwanted state switching of the second Josephson device (monitor gate). In that circuit, the trigger pulse applied to the first Josephson device must be of very short duration and of fixed amplitude, otherwise more than one sample pulse may be produced in response to a simple trigger pulse. High speed cables are also required on the input of this circuit. Further, the first Josephson device may produce different sample pulses in response to different trigger pulses, which also adversely affects circuit integrity. Still further, the monitor gate may not latch in its voltage state if the unknown signal is reversed rapidly and if the trigger pulse is not present.
In the design of an ultra fast sampling system, it is important that the switching threshold of the monitor gate be unambiguously defined, and that the resolution and sensitivity not be limited by the necessity to use commercially available equipment. Also, it is advantageous if the trigger pulse generator does not require tight tolerances, and if the system will measure both rising and falling portions of the unknown signal.
It is another object of the present invention to provide a simple sampling technique in which accurate picosecond resolution can be obtained without the need for additional fast and expensive instrumentation, and which can be extended to sub picosecond resolution.
It is another object of the present invention to provide a superconductive method and apparatus which can measure the amplitude and shape of any unknown waveform, including electrical signals, optical signals, x-ray signals, gamma ray signals, or even particle signals.
It is still another object of the present invention to provide a Josephson device sampling circuit having a timing means to establish a timing reference for the start of a measurement sample and for establishing the sampling delay times, without relying on conventional room temperature sampling means.
It is yet another object of the present invention to provide an improved Josephson device sampling circuit using presently available components and which also includes noise elimination means and timing means for establishing a timing reference.
It is another object of the present invention to provide an ultra high resolution sampling system which can accurately sample and record the rise and fall of general unknown signals.